Conventionally, a ceramic honeycomb structure has been used widely as an exhaust gas purifying filter, a catalyst carrier, and the like. In recent years, as the regulation of exhaust gas has been tightened, a honeycomb structure having a wall thickness of 120 μm or smaller, which can offer higher exhaust gas purifying performance, has been demanded strongly.
On the other hand, as a manufacturing method for such a honeycomb structure, an extrusion molding method has generally been used. When extrusion molding is performed, it is a slit width of a die that determines the wall thickness of honeycomb structure, and various dies in which a coating layer is formed on a die base to adjust the slit width have been disclosed.
Specifically, as a conventional die for molding a honeycomb structure, there has been disclosed a die in which a coating layer is formed on a die base by electroless plating to adjust the slit width, and a rounded portion having roundness is formed at a corner of face forming each slit to increase its strength (Japanese Patent Publication No. 61-39167).
Also, a die for molding a honeycomb structure, in which a coating layer consisting of iron boride, chromium carbide, aluminum oxide, titanium carbide, titanium nitride, or titanium carbide nitride is formed on a die base by chemical vapor deposition (CVD) to improve abrasion resistance, has been disclosed (Japanese Patent Laid-Open No. 60-145804).
However, in the die for molding a honeycomb structure described in Japanese Patent Publication No. 61-39167, since the coating layer is formed by electroless nickel plating, the abrasion resistance of the coating layer is not necessarily sufficient to accomplish extrusion of ceramic material.
Also, in the die described in Japanese Patent Laid-Open No. 60-145804, since the coating layer formed by chemical vapor deposition (CVD) has a thickness of about 30 μm at the most, if the coating layer is formed by chemical vapor deposition (CVD) after slits are provided roughly by a method such as electrical discharge machining (EDM), grinding, or the like, it is very difficult to finally form slits having a slit width of 120 μm or narrower, so that a honeycomb structure that meets the above-described recent demand cannot be obtained.
The applicant of the present invention has proposed a die for molding a honeycomb structure, in which a substrate layer is formed by electroless plating to roughly adjust the slit width, and then a surface layer is formed by chemical vapor deposition (CVD), by which slits having a slit width of 30 to 200 μm are provided (Japanese Patent Laid-Open No. 10-309713).
However, according to the subsequent study conducted by the inventor, it was found that in this die for molding a honeycomb structure, when or after the surface layer is formed by chemical vapor deposition (CVD), the substrate layer easily peels off, which sometimes results in damage to the surface layer.